Hemostatic surgical techniques, in which heated instruments are used to thermally reform the collagen of incised tissue during surgery, are well known. For example, heated scalpel blades have been used that prevent bleeding by causing hemostasis of tissue as it is cut. It is frequently desirable to operate such surgical instruments at a selected operating temperature, depending upon the type of surgery to be performed.
Typically, resistively-heated surgical instruments are powered by an adjustable power supply to achieve a desired operating temperature. The operating temperature of the blade is generally adjusted by selecting an appropriate power level on the power supply. However, with previously known power supplies, it is often difficult to maintain a constant blade temperature, as the thermal load placed on the instrument causes the blade temperature to fluctuate considerably during a surgical procedure.
For example, as the blade of the surgical instrument passes through fresh tissue, the temperature of the blade may drop, thus requiring the power supply to increase the power supplied to the surgical instrument to maintain the selected operating temperature. As the temperature of the surgical instrument fluctuates, the thermal energy deposited by the surgical instrument in the adjacent tissue therefore also varies, and may result in uneven or ineffective hemostasis.
It would therefore be desirable to provide a power supply that enables an operator to select a desired instrument operating temperature, and that monitors the instrument temperature and adjusts the power that is delivered to the instrument to maintain a substantially constant instrument operating temperature under varying thermal loads.
It is known that the application of electrical power to a resistively-heated surgical instrument may result in current leakage to the patient. Where the current is supplied in the form of a direct current voltage waveform, or an alternating-current waveform having a frequency of less than about 100 kHz, undesirable neuromuscular stimulation of the patient may occur. Such stimulation may interfere with the surgeon's ability to manipulate the surgical instrument, and may also injure the patient. On the other hand, it is known that precise control of the temperature of a resistively-heated heating element can be achieved using a power supply that supplies direct current waveform.
It would therefore be desirable to provide a power supply that provides the safety features inherent in using a high frequency alternating-current signal, but with the temperature control achievable with a direct current power supply.
Tamura U.S. Pat. No. 4,549,073 describes a DC power supply for use with resistively-heated heating elements. The power supply described in that patent employs a fixed DC sense current to derive a heating element resistance. While the device described in that patent provides temperature regulation of the heating element, supply of the DC drive current to provide power to the heating element must be interrupted during the resistance measuring operation. Thus, the heating element temperature cannot be measured during the heating process. Further, the circuitry used to switch between the DC power signal and the DC sense circuit adds complexity and additional cost to the design of the supply. Additionally, as discussed above, the power supply uses undesirable DC power to generate heat in the heating element.
It would therefore be desirable to be able to provide a power supply that allows the heating element temperature to be monitored while simultaneously powering the heater yet does not employ DC power to heat the heating element.
Tamura et al. U.S. Pat. No. 4,523,084, Japanese Utility Model Publication No. 51-15160 and Japanese Utility Model Publication No. 51-110615, also describe power supplies for use with resistively-heated heating elements. However, those reference also employ undesirable DC power to heat the heating element.